1. Field of the Invention
The present invention relates to a carburetor choke mechanism which controls a degree of opening of a choke valve automatically.
2. Related Background of the Invention
A choke mechanism is used in order to enhance an air-fuel ratio temporarily at the time of engine start-up. Although a choke valve has been opened and closed by manual operation heretofore, increasing recently has been an automatic choke type where the degree of opening of the choke valve provided in a carburetor has been made to be dependent on a change of an engine temperature for achieving stabilizing of a warming-up operation, and enhancing in fuel efficiency (for example, Patent Document 1).
FIG. 7 is a side view showing a conventional choke mechanism shown in Patent Document 1. In addition, FIG. 8 is a sectional view of FIG. 7, and FIG. 8A shows an A-A cross section, and FIG. 8B shows a B-B cross section.
The choke mechanism described in Patent Document 1 is designed to have a mechanism in which a valve shaft 31a of a choke valve 31 is arranged at a position which is offset to one side from a center line of an intake-air path 30a, and the choke valve 31 inclines against the center line of the intake-air path 30a so that a large side of the choke valve 31 rather than a small side thereof may be located at a downstream side of the intake-air path 30a in a fully closed state. A choke lever 33 is mounted in a form that it projects outside from a carburetor main body 30 of the valve shaft 31a, and this choke lever 33 is comprised of a bottomed cylindrical part 33a fitted to the valve shaft 31a in a rotatable state, and of a lever arm 33b protruded in a integrated form on an outer side of this bottomed cylindrical part 33a. In an inner side of the bottomed cylindrical part 33a, a pair of stopper protrusions 34 and 34′ arranged at a certain interval in the circumferential direction are formed, and a valve shaft lever 35 has been fixed to the valve shaft 31a so that the valve shaft 31a may become rotatable only between these stopper protrusions 34 and 34′. Then, a valve shaft spring 36 to energize this valve shaft lever 35 so as to be abutted on one stopper protrusion 34 located at a closing side of the choke valve 31 is provided between the choke lever 33 and the valve shaft lever 35.
At a lower outer circumference of the choke lever 33, a pair of stopper walls 37 and 37′ which are arranged at an interval in the circumferential direction are formed, and a stopper pin 38 arranged between these stopper walls 37 and 37′ is protruded from the carburetor main body 30. Then, by one stopper wall 37 abutting on the stopper pin 38, a closing position C of the choke lever 33 at which the choke valve 31 is made to be fully closed is specified, and by the other stopper wall 37′ abutting on the stopper pin 38, a opening position O of the choke lever 33 at which the choke valve 31 is made to be fully opened is specified.
Opening and closing of the choke valve 31 in the case of an automatic choke type is performed by using an actuator which is expanded and contracted depending on the change of the engine temperature. However, in the case of the choke mechanism like Patent Document 1, at the time of fully closing or small degree of opening of the choke valve 31, if an engine intake-air negative pressure exceeds a certain value, a difference between a rotation moment caused by the intake-air negative pressure which operates on the large side of the choke valve 31 and a rotation moment caused by the intake-air negative pressure which operates on the small side of the choke valve 31 overcomes a rotation moment caused by the valve shaft spring 36, and the degree of opening of the choke valve 31 may be increased. That is, the choke valve 31 is energized to a fully-closed side, and however, it has such a configuration as to be able to be opened without depending on an actuator when the intake-air negative pressure becomes large. Besides, the degree of opening thereof is limited by the valve shaft lever 35 being abutted on the other stopper protrusion 34′.